A highly efficient and environmentally benign synthesis of 6,8-dibromoflavones, 8-bromoflavones, 5,7-dibromoaurones and 7-bromoaurones

Various ring substituted 6,8-dibromoflavones (3a–e) as well as 8-bromoflavones (3f–j) can be synthesized easily from the corresponding 2′-hydroxychalcones (1a–j) in good yields and in two steps under environmentally benign reaction conditions. This can be achieved by bromination with concomitant cyclization by using a combination of vanadium pentoxide, hydrogen peroxide and ammonium bromide in dichloromethane-water at 0–5 °C, followed by dehydrobromination of the brominated products 2a–j using 0.2 M ethanolic KOH solution at ice-bath temperature. On the other hand, various 5,7-dibromoaurones and 7-bromoaurone derivatives 6a–c can be obtained, exclusively, from the corresponding 2′-acetoxychalcones (4a–c) in good yields and in two steps by tuning the reaction conditions.